<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span></span></div>
<script>
  function f(){
    let i = document.querySelector("#i1");
    let s = document.querySelector("span")
      /*isNaN()判断变量是否是NaN*/
      if(isNaN(i.value)){
          s.innerText="输入有误!"
          return;
      }
      /*s.innerText = i.value*i.value;*/
      //当两个字符串进行-*/运算时会自动转成数值,进行+运算是拼接
      //s.innerText = i.value*1+i.value*1
      //parseFloat()将字符串转换成整数或小数
      s.innerText = parseFloat(i.value)+parseFloat(i.value);
  }
</script>
</body>

</html>